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An analyst is reviewing the Notable Senders section in Proofpoint Supplier Threat Protection.
Based on the data shown in the exhibit, which vendor's email activity should be investigated first?
Supplier Threat Protection prioritization focuses on vendor identities whose messaging patterns indicate elevated risk---such as unusual sending behavior, higher malicious/suspicious message counts, abnormal spike patterns, or stronger impersonation/compromise indicators relative to other suppliers. Based on the exhibit's Notable Senders metrics, bob@aerowestglobalservices.com (C) shows the highest-risk activity and should be investigated first. In Proofpoint IR workflow, supplier-related threats are high impact because they exploit trust relationships and can bypass user suspicion (invoice/payment workflows, shared documents, ongoing threads). The investigation typically validates whether this is: (1) a compromised supplier mailbox, (2) supplier-domain impersonation (lookalike domain), or (3) a legitimate supplier system misconfigured and sending risky content. Analysts pivot into message samples, authentication alignment (SPF/DKIM/DMARC), sending infrastructure changes, and recipient targeting patterns (finance/AP, executives). If malicious, containment includes blocking the supplier sender/domain (or precise subdomains), pulling delivered copies via TRAP, alerting impacted users, and initiating vendor contact to remediate the supplier's account security.